Conditionally define bindings based on available version of libgexiv2

[felixc]

Upstream gexiv2 seems to be actively developed again. This means there are new symbols being defined in new releases. We should conditionally expose them if and only if they are actually available in the currently installed version of libgexiv2 (Debian Stable, for example, doesn't have everything).

References:

• https://doc.rust-lang.org/cargo/reference/build-script-examples.html#conditional-compilation
• https://gitlab.gnome.org/GNOME/gexiv2/blob/master/NEWS
• https://gnome.pages.gitlab.gnome.org/gexiv2/docs/GExiv2Metadata.html#gexiv2-metadata-try-get-tag-label (c.f. "Since 0.12.2")